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Share-based compensation - LTIP Awards Linked To Other Performance Conditions (Details) - LTIP awards linked to other performance conditions - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation, Restricted Shares [Roll Forward]      
Outstanding, beginning of period 222,647 364,731 296,843
Granted 0 0 129,603
Vested (109,097) (121,249) (65,089)
Increased above target (in shares) (1,989) 3,412 4,949
Forfeited (1,745) (24,247) (1,575)
Outstanding, end of period 109,816 222,647 364,731
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Outstanding, beginning of period (in dollars per share) $ 26.02 $ 26.01 $ 26.14
Granted (in dollars per share) 0.00 0.00 25.18
Vested (in dollars per share) 26.89 26.05 24.95
Increased above target (in dollars per share) 25.26 26.89 26.70
Forfeited (in dollars per share) 25.19 25.82 26.07
Outstanding, end of period (in dollars per share) $ 25.18 $ 26.02 $ 26.01
Total weighted-average grant-date fair value $ 0.0 $ 0.0 $ 3.3